Flip-chip assembly of protected micromechanical devices

ABSTRACT

A low-cost ceramic package, in land-grid array or ball-grid array configuration, for micromechanical components is fabricated by coating the whole integrated circuits wafer with a protective material, selectively etching the coating for solder ball attachment, singulating the chips, flip-chip assembling a chip onto the opening of a ceramic substrate, under filling the gaps between the solder joints with a polymeric encapsulant, removing the protective material form the components, and attaching a lid to the substrate for sealing the package. It is an aspect of the present invention to be applicable to a variety of different semiconductor micromechanical devices, for instance actuators, motors, sensors, spatial light modulators, and deformable mirror devices. In all applications, the invention achieves technical advantages as well as significant cost reduction and yield increase.

CROSS-REFERENCE TO RELATED PATENTS

This is a continuation of U.S. application Ser. No. 10/695,026, filedOct. 28, 2003, which is a continuation of U.S. application Ser. No.10/283,494, filed Oct. 30, 2002, which is a divisional of 09/779,001,filed Feb. 8, 2001, which claims priority to U.S. Provision ApplicationNo. 60/184,091, filed Feb. 22, 2000, the entireties of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes and more specifically to the designand fabrication of a land-grid array/ball-grid array package based onflip-chip assembly generally of micromechanical devices and specificallyof digital mirror devices.

DESCRIPTION OF THE RELATED ART

Micromechanical devices include actuators, motors, sensors, spatiallight modulators (SLM), digital micromirror devices or deformable mirrordevices (DMD), and others. The technical potential of these devices isespecially evident when the devices are integrated with semiconductorcircuitry using the miniaturization capability of semiconductortechnology.

SLMs are transducers that modulate incident light in a special patternpursuant to an electrical or other input. The incident light may bemodulated in phase, intensity, polarization or direction. SLMs of thedeformable mirror class include micromechanical arrays of electronicallyaddressable mirror elements or pixels, which are selectively movable ordeformable. Each mirror element is movable in response to an electricalinput to an integrated addressing circuit formed monolithically with theaddressable mirror elements in a common substrate. Incident light ismodulated in direction and/or phase by reflection from each element.

As set forth in greater detail in commonly assigned U.S. Pat. No.5,061,049, issued on Oct. 29, 1991 (Hornbeck, “Spatial Light Modulatorand Method”), deformable mirror SLMs are often referred to as DMDs inthree general categories: elastometric, membrane, and beam. The lattercategory includes torsion beam DMDs, cantilever beam DMDs, and flexurebeam DMDs. Each movable mirror element of all three types of beam DMDincludes a relatively thick metal reflector supported in a normal,undeflected position by an integral, relatively thin metal beam. In thenormal position, the reflector is spaced from a substrate-supported,underlying control electrode, which may have a voltage selectivelyimpressed thereon by the addressing circuit.

When the control electrode carries an appropriate voltage, the reflectoris electrostatically attracted thereto and moves or is deflected out ofthe normal position toward the control electrode and the substrate. Suchmovement or deflection of the reflector causes deformation of itssupporting beam storing therein potential energy which tends to returnthe reflector to its normal position when the control electrode isde-energized. The deformation of a cantilever beam comprises bendingabout an axis normal to the beam's axis. The deformation of a torsionbeam comprises deformation by twisting about an axis parallel to thebeam's axis. The deformation of a flexure beam, which is a relativelylong cantilever beam connected to the reflector by a relatively shorttorsion beam, comprises both types of deformation, permitting thereflector to move in piston-like fashion.

A typical DMD includes an array of numerous pixels, the reflectors ofeach of which are selectively positioned to reflect or not to reflectlight to a desired site. In order to avoid an accidental engagement of areflector and its control electrode, a landing electrode may be addedfor each reflector. It has been found, though, that a deflectedreflector will sometimes stick or adhere to its landing electrode. Ithas been postulated that such sticking is caused by intermolecularattraction between the reflector and the landing electrode or by highsurface energy substances adsorbed on the surface of the landingelectrode and/or on the portion of the reflector which contacts thelanding electrode. Substances which may impart such high surface energyto the reflector-landing electrode interface include water vapor orother ambient gases (e.g., carbon monoxide, carbon dioxide, oxygen,nitrogen) and gases and organic components resulting from or left behindfollowing production of the DMD. A suitable DMD package is disclosed incommonly assigned U.S. Pat. No. 5,293,511 issued on Mar. 8, 1994(Poradish et al., “Package for a Semiconductor Device”).

Sticking of the reflector to the landing electrode has been overcome byapplying selected numbers, durations, shapes and magnitudes of voltagepulses to the control electrode. Detail can be found in U.S. Pat. No.5,096,279, issued on Mar. 17, 1992 (Hornbeck et al., “Spatial LightModulator and Method”). Further improvement of the sticking problem isdisclosed in commonly assigned U.S. Pat. No. 5,331,454, issued on Jul.19, 1994 (Hornbeck, “Low Reset Voltage Process for DMD”). This patentdescribes a technique for passivating or lubricating the portion of thelanding electrode engaged by the deformed reflector, and/or the portionof the deformed reflector which engages the landing electrode.Passivation is effected by lowering the surface energy of the landingelectrode and/or the reflector, which is, in turn, effected bychemically vapor-depositing on the engageable surfaces a monolayer of along-chain aliphatic halogenated polar compound, such as perfluoroalkylacid. Objects do not easily, if at all, stick or adhere to low energysurfaces, which are also usually expected to be resistant to sorptionthereonto of high surface-energy imparting substances such as watervapor.

Refinements of the passivation method are disclosed in U.S. Pat. Nos.5,939,785, issued on Aug. 17, 1999 (Klonis et al., “MicromechanicalDevice including Time-release Passivant”), and 5,936,758, issued on Aug.10, 1999 (Fisher et al., “Method of Passivating a Micromechanical Devicewithin a Hermetic Package”). The method an enclosed sourcetime-releasing a passivant, preferably a molecular sieve or binderimpregnated with the passivant. Further, the method is placing apredetermined quantity of the passivant in the package just after deviceactivation, and is then immediately welding a hermetic lid (free ofpassivant during the welding process) to the package.

The described sensitivity of most micromechanical devices would make itmost desirable to protect them against dust, particles, gases, moistureand other environmental influences during all process steps involved indevice assembly and packaging. It is, therefore, especially unfortunatethat conventional assembly using gold wire bonding does not permit theremoval of any protective material from the micromechanical devicesafter wire bonding completion, so that the devices have to stayunprotected through these process steps. As a consequence, yield loss isalmost unavoidable.

Furthermore, today's overall package structure for micromechanicaldevices, based on multi-level metallization ceramic materials, andmethod of fabrication is expensive. This fact conflicts strongly withthe market requirements for many applications of micromechanicaldevices, which put a premium at low device cost and, therefore, lowpackage cost.

An urgent need has therefore arisen for a coherent, low-cost method ofencapsulating micromechanical chips and for a low cost reliable packagestructure. The structure should be flexible enough to be applied fordifferent micromechanical product families and a wide spectrum of designand process variations. Preferably, these innovations should beaccomplished while shortening production cycle time and increasingthroughput.

SUMMARY OF THE INVENTION

According to the present invention, a low-cost ceramic package, inland-grid array or ball-grid array configuration, for micromechanicalcomponents is fabricated by coating the whole integrated circuits waferwith a protective material, selectively etching the coating for solderball attachment, singulating the chips, flip-chip assembling a chip ontothe opening of a ceramic substrate, underfilling the gaps between thesolder joints with a polymeric encapsulant, removing the protectivematerial form the components, and attaching a lid to the substrate forsealing the package.

The package structure disclosed is flexible with regard to solder andunderfill materials and geometrical detail such as storage space forchemical compounds within the enclosed cavity of the package.

It is an aspect of the present invention to be applicable to a varietyof different semiconductor micromechanical devices, for instanceactuators, motors, sensors, spatial light modulators, and deformablemirror devices. In all applications, the invention achieves technicaladvantages as well as significant cost reduction and yield increase.

In a key embodiment of the invention, the micromechanical components aremicromirrors for a digital mirror device. In this case, the lid is aplate made of glass or any other material transparent to light, and theprotective material is a photoresist as used in photolithographicprocesses.

It is another aspect of the present invention to keep the sensitivemicromechanical components safely protected until the final attachmentof the lid, resulting in significantly higher assembly and process yieldand enhanced device quality and reliability.

Another aspect of the invention is to use well-controlled solder ballattachment and solder joint underfill processes, providing a packagewith low mechanical stress and necessary control for providing planarityrequirements.

Another aspect of the invention is to be applicable to single-levelmetal ceramic substrates, which can be manufactured at low cost.

Another aspect of the invention is to provide assembly and packagingdesigns and processes with flexibility to produce land-grid array orball-grid array packages.

These aspects have been achieved by the teachings of the inventionconcerning structure and methods suitable for mass production.

The technical advances represented by the invention, as well as theaspects thereof, will become apparent from the following description ofthe preferred embodiments of the invention, when considered inconjunction with the accompanying drawings and the novel features setforth in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a schematic top view of a semiconductor wafer having aplurality of integrated circuits with micromechanical components.

FIGS. 1B to 10 illustrate the flip-chip assembly process steps accordingto the invention for fabricating a land-grid array package and aball-grid array package for a chip having micromechanical components.

FIG. 1B is a schematic cross section of a portion of the semiconductorwafer in

FIG. 1A. The cross sectional view illustrates a plurality ofmicromechanical components coated by a protective material according tothe invention.

FIG. 2 is a schematic cross section of the portion of the semiconductorwafer shown in FIG. 1B after selective etching of the protectivecoating.

FIG. 3 is a schematic cross section of the portion of the semiconductorwafer shown in FIG. 2 after depositing the solder balls.

FIG. 4 is a schematic cross section of a discrete chip havingmicromechanical components and solder balls, after separation from thecomposite wafer shown in FIG. 3.

FIG. 5 is a schematic top view of the insulating substrate, its centralopening and two pluralities of metallic contact pads.

FIG. 6 is a schematic cross section of the substrate shown in FIG. 5after mounting the chip shown in FIG. 4 according to the invention.

FIG. 7 is a schematic cross section of the mounted chip shown in FIG. 6after filling the gap spacing the chip and the substrate with apolymeric encapsulant according to the invention.

FIG. 8 is a schematic cross section of the assembled chip shown in FIG.7 after removing the protective material from the micromechanicalcomponents.

FIG. 9 is a schematic cross section of the assembled chip shown in FIG.8 after attaching the lid, completing the land-grid array packageaccording to the invention.

FIG. 10 is a schematic cross section of the completed device of FIG. 9after attaching a plurality of solder balls to the substrate andcreating a ball-grid array package.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1A schematically shows a semiconductor (usually silicon) wafer,generally designated 100, with a plurality of devices 101 not yetsingulated from the wafer. A cross section along line A-A is partiallyreproduced in FIG. 1B in schematic and simplified manner.

As FIG. 1B indicates, the semiconductor (silicon) basic material 102supports a number of devices of length 104. Each device consists of anintegrated circuit (not shown in FIG. 1B) and a plurality ofmicromechanical components 103. The plurality of components 103 occupiesa length 105 located in the central portion of the total device length104. Furthermore, the plurality of components 103 is configured in aplane. In the peripheral portions of each device, there is a pluralityof metallic terminals 106, which serve as electrical inputs/outputs ofthe integrated circuit and the micromechanical components. Terminals 106are typically metallic; commonly used metals in the semiconductorindustry are aluminum, copper, and gold. Often, refractory metals suchas chromium, molybdenum, titanium, or titanium-tungsten alloy are usedover nickel, copper, or gold; they may have a surface layer ofsolder-compatible metal such as palladium or platinum.

The dashed lines designated “B” in FIG. 1B indicate the locations wherethe rotating saws will eventually separate each device from the wafer ina singulation step before assembly.

The micromechanical components 103 may include actuators, motors,sensors, spatial light modulators, and deformable mirror devices. By wayof example and purposes of illustration, micromechanical components 103are digital micromirror devices (DMD), as manufactured by TexasInstruments Incorporated, Dallas, Tex., U.S.A.

The semiconductor basic material is usually silicon insingle-crystalline form. The material may also be silicon-germanium,gallium arsenide, or any other semiconductor material used for deviceproduction.

The fabrication process flow steps of the preferred embodiment areillustrated in FIGS. 1B through 9 and 10.

Process Step 1: Coating Wafer Surface.

For the full benefit of the present invention, it is pivotally importantthat the surface of the whole wafer including the plurality ofmicromechanical components 103 is coated with a protective material 107;see FIG. 1B. By way of example, material 107 may be a layer ofphotoresist material as used in semiconductor photolithographicprocesses. This layer is able to withstand the elevated temperaturesemployed later in solder reflow and underfill curing. The protectivelayer prevents any deposition of dust or environmental particles on themicromechanical devices and protects the devices against process-relateddamages (such as scratches), until all process steps are completed.Consequently, the protective layer contributes significantly to processyield and device quality improvement.

Process Step 2: Selectively Etching.

As shown in FIG. 2, the protective layer 107 is etched selectively inorder to expose the terminals 106 of each device. The portions 107 a ofthe protective coating, however, must remain over the plurality ofmicromechanical components 103 of each device.

Process Step 3: Depositing Solder Balls.

In the next process step, illustrated in FIG. 3, solder balls 301 aredeposited onto the exposed terminals 106 so that one solder ball isdeposited onto each terminal.

As used herein, the term solder “ball” does not imply that the soldercontacts are necessarily spherical; they may have various forms, such assemispherical, half-dome, truncated cone, or generally bump, or acylinder with straight, concave or convex outlines. The exact shape is afunction of the deposition technique (such as evaporation, plating, orprefabricated units) and reflow technique (such as infrared or radiantheat), and the material composition. Generally, a mixture of lead andtin is used; other materials include indium, alloys of tin/indium, tinsilver, tin/bismuth, or conductive adhesive compounds. The meltingtemperature of the solder balls 310 may be different (usually higher)from the melting temperature of the solder balls used for connecting thedevice to the outside world.

Several methods are available to achieve consistency of geometricalshape by controlling amount of material and uniformity of reflowtemperature. Typically, the diameter of the solder balls ranges from 0.1to 0.5 mm, but can be significantly larger. Commercial suppliers forpre-fabricated solder balls are, for example, Indium Metals, Utica,N.Y., U.S.A.; Alpha Metals, Jersey City, N.J., U.S.A.

More technical detail about depositing solder balls on a whole wafer,without ball duplication or missing a ball, can be found, for instance,in U.S. patent application Ser. Nos. 09/186,973, filed Nov. 5, 1998(Heinen et al., “Wafer-Scale Assembly of Chip-Size Packages”), and No.60/066,268, filed Nov. 20, 1997 (Amador et al., “Wafer-scale Assembly ofChip-Size Packages”), to which the present invention is related.

Process Step 4: Separating Composite Structure.

Lines 302 in FIG. 3 extend through surface portions freed from theprotective coating; however, they indicate the same positions as lines“B” in FIG. 1B. The rotating saws, moving along the saw (or “scribe”)streets of the semiconductor wafer, separate each chip along line 302from the original wafer.

One such singulated chip is depicted schematically in the cross sectionof FIG. 4.

The plurality of micromechanical components 103, protected by coating107, is configured in a plane in the center portion of the chip. Theplurality of terminals 106, with solder balls 301 attached, isconfigured in peripheral portions of the chip. The chip with themicromechanical devices is thus readied for assembly onto a substrateusing the so-called “flip-chip” technology.

Process Step 5: Providing Substrate.

The preferred embodiment of the present invention uses a low-costelectrically insulating substrate made of ceramic having first andsecond surfaces, a central opening, and single-level metallization. Asan example, FIG. 5 shows the schematic top view of a substrate,generally designated 500, with square outline 501 and a central opening502 with square-shaped outline 503. The view of FIG. 5 is on the firstsurface 504 of the substrate.

First surface 504 exhibits the single-level metallization of thesubstrate. This metallization provides a first plurality of contact pads505 in proximity of the outline 503 of the opening. By way of example,these contact pads 505 are shown in FIG. 5 having circular shape. Theymay, however, have square shape or any other shape. The contact pads areconnected to a network of routing lines, integral with the substrate andalso portion of the single-level metallization, yet not shown in FIG. 5.

Further, first surface 504 has a second plurality of contact pads 506remote from opening 502. In FIG. 5, the pads 506 are shown in squareshape arranged in arrays of contact lands positioned along the outline501 of the substrate 500. Different geometrical shapes and arrangementsare acceptable. Contact pads 506 are also electrically connected to therouting lines (not shown in FIG. 5). Commonly used metals for bothcontact pads 505 and 506 include nickel, copper, and gold. They may havea surface layer of solder-compatible metal such as palladium orplatinum.

FIG. 6 illustrates a cross sectional view of substrate 500 along linesC-C in FIG. 5. The cross section through the ceramic material isdesignated 601. In addition to first substrate surface 504, FIG. 6 showsthe second surface 602. First surface 504 and second surface 602 aresubstantially parallel to each other. Further shown in FIG. 6 are crosssections through the first plurality of contact pads 505 and the secondplurality of contact pads 506.

Process Step 6: Aligning Chip and Substrate.

An individual chip with solder balls 301, as shown in FIG. 4, is flippedand aligned with the first plurality of contact pads 505 of substrate500. Since the configuration of solder balls 301 mirror images theconfiguration of contact pads 505, each solder ball 301 can be placedinto vertical alignment with its respective contact pad 505. Two camerassupply the vision system for alignment so that the alignment can beperformed automatically; however, a microscope for visual inspection maybe substituted. Alignment may be accomplished, for example, by rotatingand translating the chip. Flip-chip alignment to substrates is performedroutinely in industry. More detail of alignment techniques can be found,for instance, in the U.S. patent applications cited above.

Process Step 7: Forming Solder Joints.

As illustrated in FIG. 6, chip 610 having solder balls 301 is broughtinto contact with the substrate 601 having contact pads 505 so that thesolder balls 301 impact their respective contact pads 505 on thesubstrate. Next, thermal energy is applied to chip and substrate,preferably rapidly regulated radiant heat. The heating step may beperformed, for example, in an inert gas environment, such as drynitrogen or filtered gases, to provide for additional process controland to prevent excess particulates from settling on the chip surface.Non-contact or contact style thermocouples with closed-loop feedback tothe heating source may monitor the temperature on both the chip and thesubstrate.

For some micromechanical devices such as micromirrors, it may beimportant to perform the solder reflow process step using controlfeatures as described in the above-cited U.S. patent application Ser.Nos. 09/186,973 and No. 60/066,268. One important feature is to performthe alignment and heating steps in a single apparatus and in a singleoperation without moving and without the risk of losing the alignment,as could easily occur in a conventional chain furnace heating operation.The heating step follows and is combined with the alignment step, anddoes not just represents the heating of pre-assembled parts. Inaddition, the use of radiant energy sources, as opposed to furnaces,allows rapid temperature ramping or profiling, and also more uniform andmore easily controllable heating and cooling cycles. Radiant heatingallows a smooth transition from ambient temperature to the desired hottemperature, and rapid thermal response. The radiant energy ispreferably provided by an optical heat source emitting near infraredlight, such as incandescent lamps (halogen lamps with tungsten filamentand xenon filling).

Furthermore, by using selected reflective surfaces on non-active areasexposed to the near-infrared light, the assembly of chip and substratecan be heated while the remainder of the surfaces stay at a much lowertemperature. Consequently, the assembly rapidly heats up to atemperature at which solder balls 301 begin to melt or reflow. Thistemperature is typically about 183° C. During the reflow, the solderwill form a metallurgical bond (so-called “solder joint”) with the topmetal of the contact pads 505.

Another control feature, especially important for micromechanicaldevices such as micromirrors, concerns the uniformity of the height ofthe molten solder balls. It is advantageous to employ controls similarto the mechanisms described in the above-cited U.S. Patent ApplicationNo. 60/066,268. In this apparatus, three ultra-precision, independent Zaxes are arranged 120° apart and together control the Z height, Pitchand Roll of the substrate. The first step is to move the substratetowards the solid solder balls using all three axes, until the substratemakes contact with the balls. The coplanarity (pitch and roll) of thewafer to the balls is obtained by allowing each of the Z motors toindependently “bottom out” against the plane of the solder balls. The“touchdown” of the wafer on the balls can be detected by the Z motorcontroller as a sudden change in the speed of descent of the axis. Next,the pre-determined temperature profile is carried out. During theprofile, at the time by which all the solder balls should be molten, theZ axis position is reduced to a height which is equal to the balldiameter minus the known variation of the ball diameters. This actionguarantees that even the smallest diameter ball is in contact with thewafer. The diameter of the smallest ball is contained in the statisticalvariation and the ball diameter consistency as supplied by the vendor ofthe balls. Once it has been established that all balls are in contactwith the wafer and sufficient time has passed so that all balls shouldbe molten, the Z height is raised to the level at which the solder ballsare desired to be solidified, the final ball stand-off height. At thisheight the temperature is reduced to below the solder reflow temperatureand the solder balls all solidify. The height of all the solder ballswill now be equal, independent of the shape and volume of the balls. Thepreferred height of the solidified solder bumps is between 25 and 150μm, often approximately 100 μm.

As a consequence of the uniform height of the solder joints, thesubstrate 601 in FIG. 6 is positioned in a plane parallel to the planeof the micromechanical components 103. Specifically, the second surface602 of substrate 601 is in a plane parallel to the components plane.

As a further consequence of the uniform height of the solder balls, agap is spacing apart chip 610 and substrate 601. The height of the gapis equal to the height of the solder balls, and the width of the gap isequal to the distance between the solder balls.

Process Step 8: Filling Gap.

In order to form a continuous frame of material around the perimeter ofopening 502 of the substrate the gap spacing apart chip 610 andsubstrate 601 has to be filled. As indicated in FIG. 7, the filling isaccomplished by a polymeric encapsulant 701, commonly referred to as the“underfill” material. In the preferred process, care is taken to producenot only a continuous frame of material, but concurrently to reduce themechanical stress at the solder joints. As an example, a process may beused as described in U.S. patent application Nos. 60/084,440, filed May6, 1998 (Thomas, “Low Stress Method and Apparatus of UnderfillingFlip-Chip Electronic Devices”) and No. 60/084,472, filed May 6, 1998(Amador et al., “Low Stress Method and Apparatus of UnderfillingFlip-Chip Electronic Devices”), to which the present invention isrelated.

In the preferred process, the melting temperature of 183° C. for theeutectic lead/tin mixture is overshot to about 220° C. (for about 60 to120 s within the 20 min of the solder reflow period). For alternativesolder selections, times and temperatures are suitably modified. Duringreflow, the stress in the solder joint is at level zero. During thefollowing cooling, the solder solidifies, but the assembly is kept at anelevated temperature between 80 and 140° C., preferably between 90 and100° C. In this time period, the stress increases slightly from its zerolevel to a non-critical value, well below any level which could pose arisk to structurally weak dielectric layers of the chip or to the solderjoints.

It is pivotal that the assembly not be allowed to continue the coolingprocess down to ambient temperature, but is maintained at a constantelevated temperature throughout the underfill period, which may last upto 20 min. In the underfill process, the polymeric precursor isdispensed onto the first surface 504 of substrate 601 adjacent to theperimeter of chip 610. The force of surface tension pulls the viscouspolymer into the spaces between the solidified solder bumps surroundingopening 502 and forms the meniscus 701 a towards the opening and themeniscus 701 b towards the array of contact pads 506 (see FIG. 7).

Suitable polymeric precursors are formed of a material curable bythermal or radiation energy, and are preferably composed of ananhydride-hardened prepolymer such as an epoxy resin. They usuallycontain a catalyst such as an amine compound, and fillers such as silicaor alumina. Polymeric precursors are commercially available; one exampleis by the Dexter Hysol Corporation, U.S.A., under brand name FP 4527.

After completing the underfill process, the assembly proceeds directlyfrom the elevated temperature mentioned above to the increasedtemperature needed for polymerizing (“curing”) the underfill precursor.During this to time span (about 60 to 120 min), the stress falls to verylow levels. After the encapsulant his fully cured, the temperature isallowed to drop to ambient temperature in the cool down period, whilethe stress increases only slightly, well below any risk for damage tostructurally weak dielectric films or solder joints. As intended, thestress throughout the assembly is approximately uniformly distributedand for the most past absorbed by the encapsulant.

Process Step 9: Removing Protective Material.

As indicated in FIG. 7, the protective material 107 still remains overthe surface of the micromechanical components during the underfillingprocess. After ambient temperature is reached, this protective materialcan be safely removed (see FIG. 8) so that the surfaces 103 a of thecomponents 103 are exposed. When the protective material consists ofphotoresist as commonly used in semiconductor technology, the step ofremoving comprises dissolving the photoresist layer in standardpractice.

In the case of micromirror components, this process step also consistsof removing the photoresist under the micromirrors and activating thecomponents using plasma etch or a combination of a plasma etch/UV cureprocess to remove any residual contaminants from the mirror surfaces.

Process Step 10: Inserting Passivant.

For some micromechanical devices such as micromirrors, it isadvantageous to have ridge-like protrusions formed in the ceramicsubstrate near the components (not shown in FIGS. 6 to 10). Theseprotrusions serve the purpose of storing chemical compounds intended toremain inside the package volume after closure by the covering lid.These chemicals are typically supplied as pills or granular material andare suitable for releasing passivants continuously for the lifetime ofthe device in order to coat all contacting surfaces of themicromechanical devices. More detail about composition, operation andmethod of metered deposition can be found in the above-quoted U.S. Pat.Nos. 5,939,785 and 5,936,758.

Process Step 11: Attaching Lid.

Right after the deposition of any chemical compound, a lid 901 isattached to close the package, as shown in FIG. 9. Typically, lid 901has to be cleaned from impurities and dehydrated by baking inreduced-pressure environment before attachment. Afterwards, it isattached to the second surface 602 of the substrate 601, preferablyusing an epoxy adhesive. Temperature and time needed to polymerize theadhesive also serve to sublimate an amount of the passivant within thepackage so that the active surfaces of the micromechanical componentsare coated with at least a monolayer of the passivant.

For micromirror devices, lid 901 is a plate made of glass or any othermaterial transparent to light in the visible range of theelectromagnetic spectrum. Requirements for optical flatness of the plateare described in quoted U.S. Pat. No. 5,939,785. Care has to be takenthat attached lid 901 is in a plane parallel to the plane of theplurality of micromirrors 103.

By attaching lid 901, the second level of opening 502 is closed. Thefirst level of the opening is closed by solder attaching chip 610; allsides of the opening are closed by the frames of the solder bumps andthe underfill material. The micromechanical components are thus in afully enclosed package.

Process Step 12: Marking.

The enclosed micromechanical devices are marked with identification suchas device type and number, fabrication information, country of origin,etc.

Process Step 13: Attaching Solder Balls

The package as depicted in FIG. 9 is a so-called “land-grid array”package with contact pads 506 designed for pressure contacts servingmany customer needs. If a so-called “ball-grid array” package isdesired, solder “balls” 1001 may be attached to the substrate terminals506 in FIG. 10. The solder balls may be a conventional lead/tin alloy,or a lead-free mixture as described above. The diameter can vary widely;typical sizes range from 0.5 to 1.5 mm.

After electrical testing the land grid array/ball grid array device, thefinished micromechanical device is ready for packing and shipping.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the thickness of substrate and lid, as wellas the height consumed for flip-chip assembly can be minimized in orderto reduce the overall thickness of the device as needed for specificapplications. As another example, the invention can be extended to batchprocessing, further reducing packaging costs. As another example, thelocation of the substrate contacts to the “outside world” can be changedfrom the chip attachment surface to the lid attachment surface of thesubstrate. It is therefore intended that the appended claims encompassany such modifications or embodiments.

1. A semiconductor wafer comprising: a plurality of devices formed on a common surface of the semiconductor wafer, each device including an integrated circuit and a plurality of micromechanical components, and a plurality of terminals disposed about the periphery of said devices, serving as electrical inputs/outputs of said integrated circuit and said micromechanical components; and a protective material coating said micromechanical components, but not coating said periphery of said device where said terminals are located.
 2. The semiconductor wafer of claim 1 further comprising solder electrically connected to said plurality of terminals.
 3. The semiconductor wafer of claim 2 wherein said solder comprises solder balls respectively located on said terminals.
 4. The semiconductor wafer according to claim 1 wherein each said device is a digital micromirror device.
 5. The semiconductor wafer according to claim 1 wherein said micromechanical components are micromirrors.
 6. The semiconductor wafer according to claim 2 wherein said solder is selected from a group consisting of lead/tin, indium, tin/indium, tin/silver, tin/bismuth, solder paste, and solder-coated spheres.
 7. The semiconductor wafer according to claim 1, wherein said protective material is a photoresist.
 8. The semiconductor wafer according to claim 1, wherein said protective material is fills gaps between components in the device. 